The vast majority of vehicles currently in use incorporate vehicle communication systems for receiving or transmitting signals. For example, vehicle audio systems provide information and entertainment to many motorists daily. These audio systems typically include an AM/FM radio receiver that receives radio frequency (RF) signals. These RF signals are then processed and rendered as audio output.
Vehicle video entertainment systems are gaining in popularity among motorists who want to provide expanded entertainment options to rear seat passengers, such as children. Rear seat passengers in vehicles equipped with video entertainment systems can watch movies or play video games to pass time during lengthy trips.
Some vehicle video entertainment systems incorporate tuners capable of receiving broadcast signals in the VHF and UHF frequency bands. Such systems allow passengers to watch broadcast television, further expanding their entertainment options. However, programming is limited to local broadcast stations. In addition, picture and sound quality is limited by the analog nature of the broadcast signals. Further, signal quality may be poor in some areas, such as remote locations.
Satellite-based broadcast systems, such as Direct Broadcast Satellite (DBS), provide subscribers with digital television programming. Because the signals used by DBS systems are digital, picture and sound quality is enhanced relative to traditional analog broadcasting systems. In addition, a DBS transmitter can provide coverage for a much larger geographic area than the terrestrial-based transmitters used by analog broadcasters. For example, it is possible to travel across a large portion of the United States without needing to change channels as different metropolitan areas are entered and exited.
A conventional DBS receiver employs a satellite tracking system to detect the position of a satellite transmitter. By orienting or pointing a receiver antenna toward the detected position, good reception can be promoted. Satellite tracking systems typically produce imperfect information to effect initial antenna pointing. To identify the correct pointing angle toward the satellite, the antenna beam is swept in azimuth and elevation or in some combination of azimuth and elevation until the strongest satellite signal is received. This beam sweeping can be produced mechanically by physically moving the antenna. Alternatively, beam sweeping can be produced electronically by adjusting the phasing of the outputs of the antenna elements or segments. Electronic beam sweeping typically produces faster response times and higher achievable slew rates than mechanical beam sweeping.
Phased array antennas are commonly employed in the design of satellite tracking systems in which a low antenna profile is sought. Additionally, with phased array antennas, beam steering may be induced by applying a phase shift between the receiving segments of the antenna. Many phased array antennas incorporate arrays of slotted waveguides or patches, e.g., 1-D patch arrays, on a common microstrip feed. Applying phase shifts between the outputs of the slotted waveguides or the 1-D patch arrays implements beam steering along a rotational plane that is orthogonal to the orientation of the slotted waveguides or the 1-D patch arrays. With this technique and in this plane of movement, essentially any pointing angle required to track a satellite can be realized.
While applied phase shifts can realize a variety of pointing angles in the plane of movement, the technique is not as readily applied to realize pointing angles in other planes. In particular, in the plane described by the signal path and the orientation of the slotted waveguides or 1-D patch arrays, phase shifts cannot easily be applied between receiving and radiating elements of the antenna. The slots in a slotted waveguide or the patches on a 1-D patch array have a fixed spacing slightly above or slightly below one wavelength of the anticipated incident frequency, and it is difficult to impart variable phase shifts between these slots or patches. As a result, the slotted waveguide or 1-D patch array exhibits a frequency-dependent pointing angle that is offset from a plane orthogonal to the orientation of the slotted waveguide or 1-D patch array. This offset may cause the system to search for the satellite in the wrong direction and track the wrong satellite, resulting in significant acquisition and reacquisition delays and poor signal quality.
One approach to adjusting the pointing angle to a higher or lower value involves dividing the slotted waveguides or 1-D patch arrays into smaller segments. Variable phase shifts are then applied to the input or output ports of the slotted waveguides or 1-D patch arrays as appropriate to the desired angular change. This design, however, involves added complexity and, as a result, increased cost.
Another alternative involves splitting the phased array antenna into two subarrays along the orientation of the slotted waveguides or 1-D patch arrays and summing and differencing pattern signals. This technique results in the formation of an angle discriminant that makes it possible to track satellites and measure pointing offsets in a way not afforded with a single beam pattern. This approach has been effective in reducing tracking error. However, the approach does not afford sufficient beam visibility to compensate for poor open loop beam positioning.